In late years, although miniaturization of CMOS transistors advance, it is concerned about the miniaturization being near to the limit. Therefore, before the limit of the miniaturization of the CMOS transistors can be defeated, it is necessary to develop a new feature element based on a different operation principle.
A magnetoresistive RAM (MRAM) having a magneto tunnel junction (MTJ) as a basic structure (see: Non-Patent Documents 1 and 2) is a high performance energy-saving element utilizing a non-volatility of a ferromagnetic dot, and research and development for a practical use are carried out as one of major candidates of next-generation electronic devices (see: Non-Patent Document 3).
A magneto tunnel junction (MTJ) element, which is an important component of the MRAM, realizes a large magnetoresistance variation (TMR ratio) of several hundred percentages at room temperature due to quality improvement of MgO tunnel barrier and development of a high spin polarization material (see: Non-Patent Documents 4 and 5). Therefore, a reading technique, which is one of the basic operations of the MRAM, is meeting necessary performance. On the other hand, at present, as a writing technique, there is a magnetization reversal method (a spin torque method) based on a spin polarization current (see: Non-Patent Document 6).
Nevertheless, as shown in FIG. 13, a spin injection method using a present layered structure, in which an injection of spins is only one-dimensionally carried out, is theoretically applicable to only a very thin film electrode of a ferromagnetic layer having a thickness less than a spin diffusion length (on the order of several nm). Therefore, an accumulation magnetic energy decreases with miniaturization of an element size (=improvement of an integration degree), and spin in stability due to thermal fluctuation becomes a large problem. In addition, a Joule heat is generated due to a flow of charges raiding on a spin current. For this reason, a limitation, in which a material featuring a small saturation magnetization and a small friction efficiency must be used for an electrode, and which is different from optimization of spin conductivity in the MTJ, occurs, resulting in decline in a reduction of the magnetoresistance charge which is needed in a reading.
In not only the MRAM but also all high function spin devices, it is strongly desired that both improvement of spin thermal disturbance durability and low power consumption in a writing operation can be realized without incompatibility while maintaining high element performance (spin device performance).
The inventors have taken notice of a planar structure featuring high flexibility as an element structure, and have carried out researches on a method of generating a spin current without a flow of charges (which is defined as a pure spin current), and a method of controlling the spin current ((see: for example, Non-Patent Document 7).
FIG. 14 is a view showing a pure spin current. In the upper section of FIG. 14, spin-polarized electric currents are defined as usual electric current on which spin currents are ridden, flows of charges corresponding to a number of electrons (three electrons (e) in FIG. 14 occur, and spin currents including a ↑spin (two in FIG. 14) and a ↓spin (one in FIG. 14) flow in the same direction so that a compensated spin current is generated. On the other hand, in the lower section of FIG. 14, pure spin currents including a ↑spin electron (one in FIG. 14) and a ↓spin electron (one in FIG. 14) flow in the reverse directions to each other due to a diffusion current so that flows of charges are compensated with each other (zero), to thereby cause flows of electrons carrying only spin angular momentum. Since these pure spin currents are conducted due to diffusion from a nonequilibrium state to an equilibrium state, a concept of the generation and control on the pure spin currents is different from that of the flows of charges conducted in an electric field. The inventors have found a spin absorption effect by which the pure spin currents efficiently flow into a ferromagnetic dot featuring a strong spin relaxation (see: Non-Patent Document 8).
Also, the inventors have developed an element structure as shown in FIG. 15 (FIG. 15(A) is an SEM image, and FIG. 15(B) is a concept view thereof), and have proved that it is possible to carry out a magnetization reversal of a fine ferromagnetic dot of permalloy by injecting only the pure spin currents into the fine ferromagnetic dot of permalloy without any electric currents being made to directly flow thereinto (see: Non-Patent Document 9).
Further, the inventors have developed a control method of electrically rotating an accumulation spin vector by carrying out spin injections, using plural terminals (see: Non-Patent Document 10).